1. Field of the Invention
The present invention relates, generally, to improvements in catheters, and, in preferred embodiments, to a catheter that significantly reduces diffusion of substances through the catheter wall and, in further preferred embodiments, possibly hinders the formation of obstructions in the distal end tip of the catheter.
2. Description of Related Art
In the medical arts, catheters are used to inject fluids into or drain fluids away from the body. Catheters may be used in combination with a device such as a pump, which is implanted in the body of a patient, the pump being suitable for the programmed delivery of measured doses of a formulation. (A formulation is defined in the present disclosure as the substance being conveyed by the catheter. This substance may comprise either a mixture of different components or it may be a single, pure substance.) A typical example of such use is the intraperitoneal delivery of an insulin formulation. FIG. 1 illustrates an example of this use. As shown in FIG. 1, an implantable infusion pump (IIP) 10 is implanted in a patient below the skin and above the muscle in the abdomen. The IIP 10 will then dispense an infusion formulation (such as an insulin formulation) through the peritoneum wall 12 via a catheter 14. A lead 16 may connect IIP 10 to a sensing device (not shown) that is used to regulate the delivery of the infusion formulation.
Depending on the circumstances, it may be necessary or desirable that the IIP and catheter remain within the patient for extended time periods ranging from a number of days to a number of years. Thus, the catheter is exposed to the body fluids of the patient. Substances within these body fluids can, over time, diffuse through the catheter wall and interact with the infusion formulation within the lumen of the catheter. The diffusion of certain molecules may lead to the formation of catheter obstructions. For example, it is believed that diffusion of one or more molecules (for example, of CO2 and/or phenol) through the wall of a catheter carrying an insulin formulation may lead to precipitation and eventually to the formation of deposits that may result in catheter obstructions. In addition, obstructions often form at the distal end tip of the catheter, possibly as a result of CO2 mixing with the infusion formulation that is present near the tip, especially during the interval between pump strokes of the IIP.
An additional problem with inward diffusion of molecules through the wall of a catheter is that the molecules interact with the infusion formulation. Thus, the composition of the infusion formulation within the catheter may be destabilized. This makes it difficult to accurately control the properties, composition, and/or stability of the infusion formulation delivered to the infusion site. Along with the problem of diffusion of molecules through the wall into the catheter is the related problem of outward diffusion of molecules of the infusion formulation components through the wall of the catheter and into the patient at other than the infusion site. For example, when an insulin formulation is being conveyed by a catheter, preservatives within the insulin formulation, such as phenol and m-cresol, may diffuse out of the catheter. Again, as a result of this outward diffusion, the properties, composition, and/or stability of the infusion formulation delivered to the infusion site will be difficult to control. Furthermore, it is believed that the diffusion of phenol from the insulin formulation destabilizes the insulin formulation and has a tendency to precipitate and lead to the formation of catheter obstructions.
As the technology improves, implant devices such as the IIP are becoming smaller. Because of the smaller size of the IIP, only a smaller volume of the infusion formulation can be accommodated within the IIP. Thus, higher concentration infusion formulations are normally used. With higher concentration infusion formulations, it is even more important that the properties, composition, and/or stability of the infusion formulation are accurately controlled.
An exemplary catheter comprises a lumen which extends through an inner wall of polyethylene and a relatively flexible outer wall of bio-compatible polymer material such as silicone rubber. The lumen is the medium through which the infusion formulation will pass. The distal end of the catheter is usually located at an infusion site. The proximal end of the catheter is usually attached to a source of the infusion formulation, which may be within an IIP.
An exemplary catheter, as discussed above, may comprise an inner wall of polyethylene and an outer wall of silicone rubber. Both of these materials can be insufficient barriers to diffusion. In addition, the exemplary catheter discussed above allows a infusion formulation to be resident in the catheter for long periods of time before it is dispensed from the distal end. CO2 and other substances present in body fluids may diffuse and/or mix with the infusion formulation in the distal end tip and cause obstructions to form.
Accordingly, there is a demand for a catheter that restricts diffusion into or out of the catheter walls. There is also a demand for a catheter that will reduce the chances that the infusion formulation flowing through the lumen will be hindered by obstructions in the distal end tip of the catheter.